Reconsideration on structural anisotropy of silica hydrogels prepared in magnetic field

• The direction of magnetic alignment of silica gels is reconsidered.
• Results of size/shape of nanocrystallites in gels overturns a previous conclusion.
• Pb(II)-doped silica gel networks aligned along the magnetic field.

In a previous paper (Mori et al., 2008), we carried out birefringence measurements of Pb(II)-doped silica hydrogels prepared in a magnetic field (B). For a 5 T sample, we observed a negative birefringence with the optic axis along B. At that time, providing a positive intrinsic birefringence of silica, we speculated that in the birefringent gels the gel network extended perpendicular to B. The purpose of this paper is to reconsider this speculation on the basis of previous and recent results (Kaito et al., 2006, 2015). In the former, the silica gels were used as a medium of a crystal growth of PbBr2 and aligned arrays of crystallites with long axis parallel to B were obtained. In the latter, Pb(II) nanocrystallites were formed in silica xerogels by electron irradiation. Both of the short axis of PbBr2 crystallites and the diameter of Pb(II) crystallites were a few tens of nanometers. This size must be a size of short axis of pores in the gel networks elongated affected by the magnetic field. Since the PbBr2 crystallites were elongated along the magnetic field, we conclude that the Pb(II)-doped silica gel networks aligned along the magnetic field.

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